The present invention relates to a solid-state image sensor and, more particularly, to a solid-state image sensor comprising a semiconductor substrate having a charge transferring section and a photoconductive film or layer formed on the semiconductor substrate.
A conventional solid-state image sensor with a photoconductive film has good characteristics such as high sensitivity and low snear since the photoconductive layer converts light rays to electrical signals. This sensor can be used in various TV cameras, for instance as a monitor TV camera, and it is regarded as a solid-state sensor of the next generation.
A typical image sensor comprises a semiconductor substrate having a charge transferring section, a photoelectric conversion section formed on the substrate, and a transparent electrode formed on the photoelectric conversion section.
The semiconductor substrate has charge storage diode regions corresponding to pixels (or picture elements) and pixel electrodes connected to the diode regions. The photoelectric conversion section is formed on the pixel electrodes, and a transparent electrode is formed on the photoelectric conversion section. This section has a barrier layer at the side of the transparent electrode. The barrier layer prevents electrons from entering into the pixel electrodes from the transparent electrode. The section having a photoconductive layer and a barrier layer provides a low dark current and a small image lag. When the photoelectric conversion layer is a film made of an i-type hydrogenated amorphous silicon, the barrier layer is often made of a p-type hydrogenated amorphous silicon carbide. In this case, an i-p structure is provided in the photoelectric conversion section.
The conventional image sensor has a major problem, i.e., blooming. Blooming occurs when any pixel is illuminated with an excessively intense light ray, and hence an excessive electrical charge is accumulated. Such excess charges overflow the pixel and are inevitably transferred to the adjacent pixels. The adjacent pixels as well as the intensely illuminated pixel generate electrical signals. When an intense light ray is applied to the image sensor, a bright vertical line or lines will appear on the screen of a display apparatus, inevitably degrading the quality of the image.
Two factors promote the blooming. The first factor is that the pixel electrodes are spaced apart to the extent that the total area of the gaps between the pixel electrodes is 30% of the total light-receiving area of the image sensor. The light rays passing through the gaps among the pixel electrodes reach the semiconductor substrate and are converted into carriers. The carriers diffuse from the charge storage diode to the charge transferring section and mix with the other carriers stored in the device. The second factor promoting blooming is that the storage capacity of the substrate is several times the maximum vertical transfer charge of the semiconductor device. When intense light rays are applied to the image sensor, an excessively great charge is generated and overflows from the charge storage section.
A device having a blooming prevention structure is disclosed in Japanese patent disclosure No. 55-104176. Such a device has a light-shielding section for shielding light rays passing through the gaps between pixel electrodes. Apparently, however, the light-shielding section can do nothing against the blooming occurring due to the insufficient transfer capacity of the charge transferring section.
A method having a blooming prevention structure is known. This method is to apply a voltage to the transparent electrode so that an excessive charge is transferred from the semiconductor device to the electrode. When this method is used, a sufficient amount of charge cannot be transferred to the electrode since the barrier layer is provided in the photoelectric conversion section.
Still another device having a blooming prevention structure is known. In this device, electrodes are formed in the gaps between the pixel electrodes and connected to the photoelectric conversion section, and a voltage is externally applied to these electrodes to discharge the charge in the photoelectric conversion section. More specifically, Japanese patent disclosure Nos. 58-17784 and 58-80975 disclose image sensors having a charge control electrode.
In the image sensor of disclosure No. 58-17784, the charge control electrode is formed between the pixel electrodes to improve resolution. With this structure, it is impossible to sufficiently prevent light rays from reaching the semiconductor substrate through the gap among the pixel electrodes and the charge control electrode. When the packing density of pixels is such that each pixel is 1 .mu.m long and 1 .mu.m wide, it is difficult to form a charge control electrode between the pixel electrodes.
In the image sensor of disclosure No. 58-80975, the charge control electrode is formed in the photoelectric conversion section. However, as in the conventional image sensors, the light rays cannot be sufficiently shielded from reaching the semiconductor substrate. Hence, blooming cannot be completely prevented. Furthermore, it requires a complicated process to form the charge control electrode in the conversion section. As a result, the breakdown voltage of the conversion section is lowered, and the image sensor may be damaged.